For 100% mortality, concentrations of potassium capricate, 0.0009 to 0.005 M, are very effective insecticides against Aedes aegypti (Linnaeus), particularly larvae, in the laboratory (23.00° ± 0.1 °C) within 1 to 5 days. And likewise potassium capricate, 0.001 to 0.002 M, and variously applied capric acid per se are effective against native mosquitoes in water pools in the field (5°–16 °C). With dosage–mortality tests of potassium capricate in the laboratory, larval mortality in A. aegypti was directly dependent on concentration, time, and age, tolerance increasing with age of the larvae. Although pupae seemed little affected, about 60% of the adults formed died during eclosion in 0.003 to 0.005 M mostly within 3 days. Admittedly these substances kill fish, crustaceans, and other insects, but in the field, treated pools soon regained living inhabitants. Some evidence shows that capric acid may be the least liable to endanger fish. Advantageous properties of these long-recognized insecticides include that both kill mosquitoes, both are easily biodegraded to harmless simple components, and both are readily metabolized by mammals.